Replication of animal viruses is dependent upon numerous components that are provided by the cell. For example, the transcription of viral genes requires cellular general transcription factors as well as certain sequence-specific DNA binding proteins. Typically, animal viruses encode regulatory proteins that function through cellular transcription factors to preferentially express viral genes. A knowledge of how these viral regulatory proteins function is essential for understanding viral replication and may suggest possibilities for therapeutic intervention. We have been studying the mechanisms by which the transcriptional regulatory proteins of three pathogenic viruses activate transcription: the Tat protein of human immunodeficiency virus (HIV), the Tax protein f human T- cell leukemia virus (HTLV), and the pX protein of hepatitis B virus (HBV). The HIV Tat protein is essential for viral replication and acts in a unique fashion; it binds to nascent HIV RNA to increase both transcription initiation and elongation. Over the past period of funding we have defined minimal components required for Tat-responsiveness and have developed a new in vitro system for studying the mechanism of Tat action. We will use these to study how Tat can increase both transcription elongation and initiation. HTLV Tax and HBV pX are required for efficient viral transcription and replication, and are also nuclear oncoproteins. Over the past period of funding we have found that Tax dramatically increases the DNA binding activity of a wide variety of cellular proteins that contain a basic region-leucine zipper (bZIP) DNA binding domain. Furthermore, we have show that Tax recognizes the conserved basic region of bZIPs to promote dimerization and thereby increase DNA binding. Unexpectedly, the DNA binding specificity of the Tax=-bZIP complex is different from that of the bZIP alone. Remarkably, we have discovered that HBV pX, which is not homologous to Tax, functions analogously. Experiments are proposed to study in further detail how Tax and pX modulate bZIP DNA binding and to determine the relationship of this mechanism to viral replication and transformation. Finally, ina the course of our studies we have discovered a novel cellular activity that, like Tax and pX, increases the DNA binding activity of cellular bZIP proteins. This cellular factor, which we refer to as bZIP Enhancing Factor (BEF), may function as a cellular co-factor for Tax and pX. We will isolate a cDNA clone for BEF and use this to study its role in viral gene expression and transformation.